Refrigeration really does ruin tomatoes, according to science

Tomatoes for sale in a supermarket in Princeton, Ill., in 2014. (Daniel Acker/Bloomberg)

This week in science news you can use, an international group of horticulturalists has published an extensive study in the Proceedings of the National Academy of Sciences looking at the effects of refrigeration on flavor compounds in tomatoes.

Now, we at Speaking of Science take tomato storage very seriously (we've written about it before). But what we truly love about this study is a single sentence from the PNAS news release about it: “Chilling fruits at temperatures below 12 degrees Celsius hampers enzymes that help synthesize flavor-imparting volatile compounds, resulting in relatively fresh but insipid fruits.”

Your reporter thought the “insipid fruits” made it sound like the study had been written by a character played by Judi Dench. And since we love Englishwomen with exacting standards almost as much as we love solving life's problems with science, we figured it was a good opportunity to combine the two.

So, without further ado, here is the science of how to not ruin your tomatoes, as told by Dame Judi and her compatriots.

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The study opens with what one might call “a truth universally acknowledged”: Commercial tomatoes are often flavorless. The researchers, led by horticulturalist Bo Zhang of the University of Florida, argue that the problem stems not from the tomatoes themselves, but from the post-harvest practice of chilling fruit. Keeping tomatoes at low temperatures slows down the ripening process and prevents them from rotting, but it also interferes with chemical compounds that give tomatoes their taste.

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Zhang and his colleagues wanted to investigate the genetic basis for this phenomenon, in hopes that their findings might yield a tastier tomato.

Flavor in a fruit is determined by three things: sugars, acids and “volatiles,” the chemical compounds that are largely responsible for its aroma. Sugars and acids aren't affected much by refrigeration, but most volatiles are synthesized during ripening, which is part of the reason ripe fruit has such a strong smell.

To figure out how long a tomato needs to be chilled for volatile synthesis to be disrupted, Zhang and Co. collected a bunch of them, both heirloom and ordinary store varieties, and stuck them in a 41-degree-Fahrenheit fridge for one, three or seven days. Each tomato then got either a 1- or 3-day “recovery” period at room temperature. The shorter periods of refrigeration didn't have much impact on volatile content, but seven days significantly depleted those compounds, and neither of the recovery periods seemed to bring them back.

The tomatoes were then tested by the discerning palates of 76 volunteer consumers; the tasters rated the chilled tomatoes significantly lower than ones that had been stored at room temperature.

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Next, the researchers probed what was going on inside the cold tomatoes at a molecular level. Using RNA sequencing, they were able to figure out which genes were expressed differently when chilled. It turned out that affected genes numbered in the hundreds (the tomato genome has 25,000 genes — about 5,000 more than humans). Refrigeration set off a cascade of changes, starting with a set of cold signaling genes and moving through the ones responsible for metabolism, ripening and volatile synthesis. It also affected DNA methylation — the mechanism that cells use to control which genes get turned on and off.

This is especially problematic because, unlike sugars and acids, volatiles don't stay inside a fruit once they're synthesized — they're lost through the stem scar (the spot where the fruits were once attached to a stem). A tomato may be bursting with flavor when picked, but seven straight days of cold temperatures and no volatile production render it, as Dame Judi would say, “insipid.”

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We would like to be able to say that the scientists have come up with some ingenious solution to this dilemma. After all, refrigeration is really useful to prevent produce from rotting. But Zhang and his colleagues offer no such consolation — at least, not in this study.

So, for now, if someone tries to feed you a cold tomato, there's only one right response:

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